depends: Add 'make clean' and 'make clean-all' rules
depends: Add 'make clean' and 'make clean-all' rules

Summary:
```
It's useful to have a standard way to clean up the work done by the
depends system when testing changes to it.

The `make clean-all` rule removes build artifacts for all
supported architectures (in addition to sources/), while `make clean`
only removes artifacts for current architecture (`BUILD`).
```

Backport of core [[https://github.com/bitcoin/bitcoin/pull/12715 | PR12715]].

Test Plan:
Copy your `depends` outside of your source tree and cd to this dir.

Should remove you current arch directory, the `work` and `built`
subdirs:
  make clean
Note: my system was detected as `x86_64-pc-linux-gnu` despite the triple
for the depends was `x86_64-linux-gnu` so I had to rename it.

Should remove the directories for all architectures and the `sources`
subdir:
  make clean-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5601

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